A storage device called all flash array (AFA) using a flash memory such as a solid state drive (SSD) as a memory device is known. The AFA may improve the access performance and the like by performing control considering the characteristics of the SSD, which is different from the characteristics of a hard disk drive (HDD).
In some cases, in the storage device, in order to fill a performance gap between the memory device and a control device which controls the access to the memory device, a cache memory using a dynamic random access memory (DRAM) or the like may be included in the control device. When the storage device includes a plurality of control devices, the cache memory may be included in each of the control devices.
As for operation modes using the cache memory, a WriteBack mode (hereinafter, sometimes referred to as a “WB mode”) and a WriteThrough mode (hereinafter, sometimes referred to as a “WT mode”) are known. The term “WriteBack” used herein may be referred to as “AfterWrite” or “WriteAfter”.
The WB mode is an operation mode in which a control device writes data in the cache memory in response to a Write input/output (I/O) from a host device and returns a Write I/O response to the host device when the duplexing of cache data between control devices is completed. In the WB mode, since a situation occurs in which no data exists in the memory device and data exists in the cache memory, the cache data is duplexed between the control devices, thereby holding the redundancy of the cache data.
The WT mode is an operation mode in which the control device returns a response to the host device when the control device writes data in the memory device (e.g., SSD in AFA) via the cache memory in response to the Write I/O. In the WT mode, since data is written in the memory device as a continuation of a process related to the I/O, there is no need to hold the redundancy of the cache data, thereby eliminating a need for a duplexing process between control devices.
In the WT mode, since a disk access is performed before the status related to the I/O is responded to the host device, the response time tends to degrade as compared with a case in the WB mode. For this reason, in some cases, for example, the storage device basically operates in the WB mode and may switch the operation mode from the WB mode to the WT mode for operation when a power supply such as a battery is abnormal.
Related technologies are disclosed in, for example, Japanese National Publication of International Patent Application No. 2013-510360, Japanese Laid-Open Patent Publication No. 2006-277205, Japanese National Publication of International Patent Application No. 2013-528843, Japanese Laid-Open Patent Publication No. 2006-092120, and Japanese National Publication of International Patent Application No. 2007-528079.